The present invention relates to a heat-resisting steel improved in creep rupture strength at high temperature, and a turbine part, such as blades and bolts of steam turbines, formed from the heat-resisting steel.
The maximum steam temperature and pressure currently used for driving steam turbines are 566.degree. C. and 246 kg/cm.sup.2, respectively. The steam temperature and pressure used are expected to be increased for higher thermal efficiency. These steam conditions require the material of parts constituting a turbine to have the high-temperature strength. For the improvement of the steam conditions, therefore, materials with increased high-temperature strength are being positively developed. Such development is essential to the blades and bolts, as well as to large-sized main components such as the rotor and casing.
The blades of a steam turbine are continually subjected to centrifugal force caused by high-speed rotation. If their material lacks in high-temperature strength, the blades will possibly suffer a creep deformation and bend backward against the rotor, interfering with stationary parts at their edges. Bolts used for closing up the upper and lower casings are initially subjected to a fixed clamping pressure attributed to elastic force. Normally urged by a steam pressure which acts on the casing, however, the bolts undergo a creep deformation such that the clamping pressure thereon is gradually reduced. If the clamping pressure becomes too low to maintain the sealed condition of the casing, thereby causing leakage of steam, or if the creep deformation is accumulated, the bolts may sometimes be broken themselves.
Thus, the material for the blades and bolts used at high-temperature parts of steam turbines is required to have an excellent creep characteristic, and heat-resisting steel of a steel system has conventionally been used for the material. Generally, the heat-resisting steel is less expensive and higher in normal-temperature toughness than any other heat-resisting steel with the same high-temperature strength. In addition, the former is higher in damping capability which is essentially required to a material for blades. In order to improve the high-temperature strength of the heat-resisting steel without spoiling those fundamental features thereof, various alloying components are added to the metal to strengthen the martensitic structure and to stabilize carbonitrides, thereby maintaining the high-temperature strength and structural stability to stand long time use at high temperature. In the aspect of the manufacturing, segregation of the alloying components will directly lower the high-temperature strength of the metal and, at the same time, produce undesirable ferrite around the alloying constituents. Therefore, a remelting process is introduced to prevent such segregation for homogenization of the structure.
Conventionally, 12-Cr-Mo-V-Nb steel specifically called H46 (Jessop-Saville H46 by Jessop-Saville Ltd. or Mel-Trol H46 by The Carpenter Steel Company) and 12-Cr-Mo-V-W steel called 422 (Crucible 422 by Crucible Steel Company of America) are used as the material for blades and bolts of steam turbines. Both these materials, however, have a creep rupture time ranging from about 200 to 300 hours at 600.degree. C. and with 30 kg/mm.sup.2 load. Such a creep strength cannot meet the requirement for the increase of steam temperature and pressure to improve thermal efficiency. Thus, there is a demand for the development of steel with improved high-temperature creep characteristic.